The present invention relates to a method for molding an element having a microscopic structure, a cooling apparatus utilized for such the molding process and an optical element formed by employing such the method.
In recent years, various kinds of optical elements, such as a very high-accurate objective lens, etc., have been employed in the rapidly progressing field of the optical pick-up device. Since it is possible to speedily manufacture plural products having a uniform shape by molding such the optical elements out of a material, such as a plastic material, glass material, etc., it could be said that such the molding method is suitable for the mass production of such the optical elements.
Further, an optical pick-up device, developed in recent years, employs a laser beam emitted from a semiconductor laser diode whose wavelength is shorter than ever, and has a capability of recording and/or reproducing high-density information onto/from a recording medium, such as HD, DVD, BD, etc. As an optical element to be employed for an optical system in the abovementioned optical pick-up device, there has been well known a wave plate, which converts a polarization of the incident light to emit the polarization-converted light.
Generally speaking, the wave plate is made by sticking two artificial-quartz plates together in such a manner that optical axes of them are orthogonal relative to each other. Each of the two artificial-quartz plates has a birefringence property and is polished to a thickness established in advance corresponding to a specific wavelength. Accordingly, there has been a problem that a manufacturing cost per one element is relatively high. On the other hand, it has been well known that, even if a material itself would have no birefringence property, the material on which the microscopic shape having repetitions of microscopic protrusions in a half pitch of the wavelength of the penetrating laser beam is formed, would exhibits the similar birefringence property (set forth in Patent Document 1).
[Patent Document 1]                Tokkai 2003-248118 (Japanese Non-Examined Patent Publication)        
It is necessary, however, that each of the abovementioned microscopic protrusions should be formed in a shape of high-aspect ratio in which its height is relatively great while the repetition pitch is small. Although it is theoretically possible to form such the microscopic protrusions as a fine pitch structure, a cost increase for forming such the microscopic structure would become a major problem when a certain amount of the products is led into a mass production. Specifically, Patent Document 1 teaches that the product can be acquired by conducting the steps of: creating a mold for the fine pitch structure; press-injecting a melted plastic material into the mold; and applying a pressure to form the product. However, since the melted plastic material exhibits an interfacial force, it is practically difficult to fill the melted plastic material into such a very narrow groove, a width of which would be lower than, for instance, 1600 nm. Accordingly, it has been virtually impossible to manufacture the products having such the fine pitch structure in the conventional molding mass-production process.